It is known to provide control arrangements for maintaining a specific air/fuel ratio of the air/fuel mixture supplied to an internal combustion engine. The control arrangements receive their control variables from an oxygen measuring probe mounted in the exhaust gas system of the internal combustion engine. In general, this control is superposed on a known mixture control arrangement which coarsely precontrols the composition of the air/fuel mixture. The precondition for a trouble-free operation of such a control arrangement is that the oxygen measuring probe operates without trouble. In known oxygen measuring probes, the operational readiness is assured only after a specific operating temperature. For this reason, and for probes which are heated only by the exhaust gas of the internal combustion engine, a mixture control must be provided for the cold start and the warm running of the engine. The mixture open-loop control is replaced by a mixture closed-loop control only after reaching the probe temperature which increases with the operating temperature of the internal combustion engine.
A method and an arrangement for temperature control of an exhaust gas probe is known from U.S. Pat. No. 4,419,190. This method and arrangement convert the temperature-dependent electrical resistance of the oxygen measuring probe into a temperature signal and, thereafter, drive a heating device for heating the exhaust gas probe. In this way, the operating temperature of the exhaust gas probe can be maintained constant in a simple manner and without additional sensors and measuring leads.
The operability of the probe heater is a precondition so that the exhaust gas sensor reaches its operating temperature as rapidly as possibly and thereafter is also maintained at a pregiven temperature.
Furthermore, the exhaust gas probe can be destroyed if a current flows therethrough having a value greater than that of a permissible current.
A system for monitoring the operability of a heater for an exhaust gas probe is already introduced in U.S. Pat. No. 5,090,387. Here, the operational readiness of the exhaust gas probe is monitored and a conclusion is drawn as to its operability in dependence upon the time-dependent response between switch-on of the probe heater and the detection of the operational readiness.
However, if a current flows through the exhaust gas probe having a value above a permitted range, this is not immediately detected by the system of the mentioned application but only when there is a possible complete malfunction of the exhaust gas probe.